This invention relates to magnesium desulfurization of molten ferrous metal by a novel process which achieves maximum magnesium desulfurization efficiency and substantial elimination of sulfur reversion from a slag back to the molten metal during casting thereof. Although not so limited the invention has particular utility in desulfurizing molten cast iron from a blast furnace prior to charging into an oxygen steel converter such as a basic oxygen furnace (BOF). The specification for steel produced in a BOF is presently 0.015% maximum sulfur.
In a typical system for such an operation molten cast iron is tapped into a transfer vessel such as a torpedo (or bottle) car. The metal flows through open runners from the blast furnace into the car, and some furnace slag is usually carried into the car. After the car is filled, it may be moved to a desulfurization station where desulfurizing agents are injected into the molten metal. The car is then transported to another station where it is emptied into a ladle. Slag is skimmed from the ladle, and the melt is then charged into a BOF.
Alternatively, the torpedo car may be moved after filling directly to a ladle station, and desulfurization may be conducted in a ladle after the car is emptied into it.
Cast iron made in a blast furnace has a silicon content within the range of about 0.5% to about 1.5% and sulfur about 0.02% to about 0.1%. Some of the silicon oxidizes to silica (silicon dioxide) in the open runners during tapping. The refractory used in the runners is usually silica, some of which erodes and is carried into the torpedo car, where it becomes part of the slag. Accordingly, even though the blast furnace slag which is carried into the car initially has a high sulfur capacity, the additional silica which gets into the slag during tapping normally causes the final slag cover, after the car is filled, to have a low sulfur capacity.
A major problem in the prior art practice described above is removal of all the cover slag when the torpedo car is emptied into the ladle. Even if the car can be rotated 180.degree., some slag solidifies and sticks to the inner walls of the car. If desulfurization has been conducted in the car, the slag has a high sulfur content, and this carry-over slag thus contaminates the next charge of molten cast iron when the car is returned to the blast furnace and refilled.
Sulfur reversion can thus result from the carry-over slag in the torpedo car. In practice, excess magnesium must be added to remove this sulfur. In addition, the problem of sulfur reversion can occur after desulfurization either in the car or ladle if the slag has a low sulfur capacity. This is the case when the slag is already high in sulfur as a result of carry-over slag in the car.
In conventional practice, the problems outlined above result in uncertainty regarding the amount of sulfur in the carry-over slag. This uncertainty in turn makes it difficult to predict accurately the amount of magnesium which should be added for desulfurization. Accordingly, heats having unacceptably high sulfur are produced relatively frequently, and these must be reblown in the BOF with consequent added processing cost. The alternative of adding magnesium substantially in excess of the predicted amount also increases costs and can lead to processing difficulties resulting in lower yields.
U.S. Pat. No. 4,341,554, issued July 27, 1982 to P. J. Koros et al, discloses a process for desulfurizing molten steel which comprises covering the melt with a synthetic slag layer, adding particulate lime to cover the synthetic slag, the lime being of a size such that substantially all is retained on a No. 80 sieve, injecting powdered lime into the melt along with a desulfurizing agent which vaporizes under the pressure and temperature conditions within the melt, and permitting the powdered lime to rise to the surface of the melt and form together with the particulate lime a crust which deters entry of ambient air into the melt. Preferred desulfurizing agents are magnesium and calcium silicon. The purpose of adding a particulate lime cover and for injecting powdered lime along with the desulfurizing agent is to eliminate the need for a mechanical cover over the ladle.
U.S. Pat. No. 4,374,664, issued Feb. 22, 1983 to T. Mitsuo et al, discloses a process for desulfurization of molten pig iron by addition of aluminum powder and lime, alumina or both, whereby to reduce the splashing associated with the addition of aluminum alone. The amount of aluminum added is sufficient to result in an aluminum content in the pig iron in weight percent of 0.01-0.1 times the concentration of silicon in the molten pig iron plus 0.2-1.0 times the concentration of sulfur in weight percent to be removed from the molten pig iron. The addition of aluminum prior to desulfurization is alleged to be for the purpose of improving the poor desulfurization efficiency of lime by preventing formation of high melting point shells of calcium silicate on the surfaces of the lime particles, derived from the silicon in the molten pig iron which is oxidized on the surfaces of the lime particles.
R. C. Sussman and A. M. Smillie presented a paper at the Chinese Iron and Steel Society Conference on Injection Metallurgy, Shanghai, China, Nov. 1, 1982, entitled "Progress In Hot Metal And Steel Desulfurization By Injection At Armco". This article summarizes the prior practice of injection of lime and magnesium for desulfurization of blast furnace iron and the difficulties resulting from this practice at lime-to-magnesium ratios ranging from 10:1 to 4:1. These difficulties resulted in elimination of lime from the process and use of magnesium alone as a desulfurization agent. The importance of draining torpedo cars in order to prevent sulfur reversion on the next cast is mentioned.
A paper was presented by A. M. Smillie and R. A. Huber in March 1979 to the 62nd National Open Hearth and Basic Oxygen Steel Conference entitled "Operating Experience At Youngstown Steel With Injected Salt Coated Magnesium Granules For External Desulfurization Of Hot Metal". This paper summarizes data from a mill indicating that the sulfur content of hot metal received by the steel plant was about 0.008% higher than the cast analysis and that samples of carry-over slag taken from the transfer ladle revealed a great decrease in both sulfur content and base:acid ratio. The sulfur reversion problem is thus recognized, and further data are given indicating an increase in efficiency by use of salt coated magnesium granules instead of magnesium coke and 75% magnesium-25% aluminum powder used previously.
A lecture was given by A. M. Smillie at McMaster University, Hamilton, Canada, in May 1984 entitled "External Treatment Of Hot Metal". This summarizes prior art processes, equipment and desulfurizing agents. Advantages and disadvantages of the various injection processes are discussed.
An article by O. Haida et al entitled "Injection Of Lime Base Powder Mixtures To Desulfurize Hot Metal In Torpedo Cars" in Proceedings of Scaninject II, pp 20:1-20 (June, 1980), discusses replacement of a calcium carbide desulfurization process by a lime desulfurization process. The problem of high sulfur in the carry-back slag in the torpedo car is recognized, and this is stated to amount to about 0.008% to 0.010% sulfur reversion when using the calcium carbide process. However, when using the lime process, desulfurization (i.e., negative sulfur reversion) was obtained on the order of 0.002%-0.003% sulfur. The expedient of completely deslagging a torpedo car, with consequent saving in carbide consumption, is stated to be more than counterbalanced by the labor costs and metal loss inherent in deslagging. Accordingly, even with the alleged improvement achieved with lime desulfurization, the torpedo car slag contains about 4% sulfur before desulfurization, compared to a blast furnace slag sulfur content of about 1%. There is thus no recognition in this article of the benefit to be derived from providing a fluid, high sulfur capacity slag in a transfer vessel, prior to desulfurization.
Other prior art of which applicants are aware disclose the use of aluminum, magnesium and/or lime as a desulfurizing agent.
Despite the above-discussed modifications in processing and equipment, there is still a need to minimize sulfur reversion, to increase magnesium efficiency, to improve the end point predictability of magnesium injection and to improve yields.